A known problem in high-pressure discharge lamps in general is the ignition of these lamps. Dependent on the type of lamp, a relatively high ignition voltage is required, which is generally supplied in the form of one or more ignition voltage pulses to the lamp by a starter. In practice, there may be an inadmissibly long ignition time, even when the ignition voltage pulses are sufficiently high, while furthermore a large spread of this ignition delay is obtained. This is the result of a shortage of primary electrons in the discharge vessel, introducing the lamp discharge during ignition. By adding a small quantity of a radioactive element 85Kr in the discharge vessel, the shortage of primary electrons can be eliminated so that the ignition time will become shorter and its spread is reduced. 85Kr has the drawback that it is radioactive, and its use can be avoided by using an UV enhancer. This is a relatively small discharge vessel that produces UV radiation and is placed in the proximity of the discharge vessel of the lamp. When the lamp is ignited, the UV radiation emitted by the UV enhancer ensures that there are sufficient primary electrons in the discharge vessel of the lamp.
A lamp of the type described in the opening paragraph is known from WO98/02902 (U.S. Pat. No. 5,811,933). The known lamp is a high-pressure discharge lamp, more particularly a metal halide lamp. This lamp has a discharge vessel with two lamp electrodes. The material of the discharge vessel may be quartz glass or a ceramic material. In this description and the claims, a ceramic material is understood to mean a densely sintered polycrystalline metal oxide, such as aluminum oxide or yttrium aluminum garnet, or a densely sintered polycrystalline metal nitride such as aluminum nitride. An outer envelope supporting a lamp cap surrounds the discharge vessel. The space between the discharge vessel and the outer envelope accommodates an UV enhancer, which has a wall of ceramic material and is provided with an enhancer electrode, which is connected to a first lamp electrode, and with a capacitive coupling. This capacitive coupling is realized by placing the UV enhancer in the proximity of a supply wire to a second lamp electrode. The use of a capacitively coupled UV enhancer as compared with an enhancer with two internal electrodes has the advantage that the enhancer is only operative when this is necessary, namely during the start phase of the lamp when ignition voltage pulses having a relatively high voltage and a high frequency are presented. Consequently, the enhancer does not consume energy during operation of the lamp and thus has a very long lifetime.
The use of a ceramic material for the wall of the UV enhancer has a favorable influence on the ignition behavior of the lamp, because the UV radiation generated by a ceramic UV enhancer appears to considerably increase the possibility of introducing the lamp discharge (lamp breakdown). However, the known lamp has the drawback that the UV enhancer itself is relatively difficult and relatively expensive to manufacture.